In digital microwave radio communication, a cross-polarization scheme is used. In the cross-polarization scheme, two polarizations that are orthogonal to each other (cross-polarizations), for example, a vertical polarization (V-polarization) and a horizontal polarization (H-polarization), are modulated with different signals to transmit different pieces of information, thereby achieving effective use of a frequency. However, when the cross polarization scheme is employed, if there is propagation disturbance such as rain, anisotropy in a medium occurs, and thus interference may occur in each polarization signal. For this reason, when communication is performed using the cross polarization scheme, it is necessary to perform cross-polarization interference compensation on signals received by receivers (for example, see Patent Document 1). Here, the cross-polarization interference compensation is a process of removing an interference component caused by a polarization signal (interference signal) that crosses a polarization direction of a polarization signal (main signal) to be extracted, from the received signals.
When cross-polarization interference compensation is performed on a polarization signal from one of transmitters (hereinafter referred to as a self-polarization side) among two polarization signals received by receivers, it is necessary to equalize a path length of a path of an interference wave from another transmitter (hereinafter referred to as a different-polarization side) to a demodulator on the self-polarization side with a path length of a path of an interference compensation signal from the transmitter on the different-polarization side to a cross-polarization compensator on the self-polarization side. Thus, radio communication equipment needs to perform the following operation in order to adjust a delay time difference based on the path difference between the polarization signals.
First, in order to know a delay time difference based on a path difference, one of transmitters of an opposing station apparatus outputs a polarization signal, receivers of both polarizations on a reception side receive the polarization signal, and the receives measures the phase difference between output signals of the both receivers. Next, a cable from the receiver on the different-polarization side to a cross-polarization interference compensator is processed based on the measured phase difference, thereby adjusting its length to reduce the delay time difference.
By performing such an operation, a delay time difference based on a path difference between polarization signals is adjusted.
However, this operation has a problem in that the cable processing operation in a station is complex. In order to resolve the complexity of the cable processing operation, Patent Document 2 discloses a technology for adjusting a delay time difference based on the path difference between polarizations without performing the cable processing operation in a station.
In a cross-polarization interference compensation scheme disclosed in Patent Document 2, first, transmitters of both polarizations in an opposing station apparatus outputs the same signal. Next, a signal (interference signal) of the different-polarization side among signals received by receivers of a local station is demodulated using a carrier wave and a clock signal which are synchronized with those of a signal (main signal) of the self-polarization side, and the demodulated signal is converted into an interference compensation signal using a transversal filter. This interference compensation signal is delayed on a bit-by-bit basis by a shift register. At this time, a phase comparator receives the interference compensation signal delayed on a bit-by-bit basis by the shift register and the signal of the self-polarization side, compares their phases with each other, controls the shift register based on the comparison result, and changes the number of stages in the shift register so that the phases become identical. Then, an adder adds the interference compensation signal delayed on a bit-by-bit basis by the shift register to the signal of the self-polarization side, thereby performing cross-polarization interference compensation.
With the cross-polarization interference compensation method disclosed in Patent Document 2, path lengths of both polarizations can be adjusted to be identical, and it is possible to simplify an adjustment operation of radio apparatus equipment in a station.